This "game" has several flaws. i'll list a few.
* There's a severe lack of diversity.
The player will get bored after two minutes. Add some extra problems, like addition, subtraction, division, modulo, etc. But "Times Table Challenge" by itself is hardly a game. And adding difficulty levels that don't seem to do anything is not diversity.
* No visual queue of whether the answer was correctly guessed.
How about, as a tacky example, turning the background green for correct answers, and when an answer is incorrectly, make it red.
* Bad graphic design
For example, the "Try Again?" button is not even close to centered. You have a powerful IDE for developing games at your fingertips. How about using it to properly design your buttons?
* No checks on user input.
Ex. 1. I can leave the username field blank, and it names me "<INPUT NAME>". How about making a default value, like "Johnny"? How about putting an array of names in your code and having it select randomly from the list in the event that (username == "")?
Ex. 2. I shouldn't be able to spam-click the answer button. Why is there no check for (answer == "")?
* Score builds up throughout game without ever being reset.
The fact that the game wants me to "try again" is a pretty good hint that it wants me to start over. The fact that the score just keeps on accumulating points throughout all plays is just plain absurd. Obviously, this is not right. If you want to keep track of "all time scores", either make an array of all scores and make it available through some "stats" menu, or just keep an individual "score" and "high score"
* Wrong answers don't cause point penalty
How can I have 550 points when I have 55 correct answers and over 300 wrong answers from spam-clicking? Shouldn't the wrong answers count against my score?
* Can't play via keyboard.
It's not hard to implement. Really, it's not. There's an entire class called Key that is devoted to making it as simple as possible.
* Timer ticks at an interval of about 2 ticks per second.
Obviously, this is way too fast. The general assumption here is an interval of one second. To make it tick twice a second is non-sensical. If you're using getTimer(), don't. Use setInterval() or setTimeout() instead.
Overall, this game feels like something that you made in less than an hour. Am I wrong? Did you take longer?
This game deserves a 1.5/5 in its current state.